Theories%20of%20Anaesthesia

1
Theories of Anaesthesia

• Dr. Pratheeba Durairaj,MMC
• 14.11.08

2
Introduction

• The mechanisms of anesthesia are surprisingly
  little understood
• Anesthetics are unique drugs in pharmacology.
  They affect all macromolecules.
• The diversity of the structures of these
  molecules indicates that there are no common
  receptors.
• The action of anesthetics is nonspecific and
  physical

3
Why should we search for mechanisms of
anaesthesia ?

• The safety of anaesthetics has improved over the
  years even though we still are to find exactly
  how they act.
• Search of molecular mechanisms may lead to
  development of safer drugs with less adverse
  effects

4
(No Transcript)
5
Cell fat dissolution theory

• Von Bibra and Harless, in 1847, were the first to
  suggest that general anaesthetics may act by
  dissolving in the fatty fraction of brain cells.
• They proposed that anaesthetics dissolve and
  remove fatty constituents from brain cells,
  changing their activity and inducing anaesthesia.

6
Colloid theory

• 1875 CLAUDE BERNARD
• Studied anaesthetic induced inhibition of
  protoplasmic streaming in slime moulds
• Proposed that a reversible coagulation of cell
  colloids accompanied anaesthesia

7
Meyer and Overton hypothesis

• 1899 /1901 - Lipid solubility theory - H. H.
  Meyer / Overton.
• The most striking correlation observed 100 yrs
  ago between the physical properties of general
  anaesthetic molecules and their potency.
• States that narcosis occurs when critical drug
  concentration is achieved in crucial lipid of CNS
• Suggests that when an anaesthetic dissolves in a
  liphophilic portion of membrane, blockade of
  essential pore ? Sodium channel occurs preventing
  depolarisation

8
Contd

• Meyer
• compared the potency of many agents, defined as
  the reciprocal of the molar concentration
  required to induce anaesthesia in tadpoles, with
  their olive oil/water partition coefficient.
• Nearly linear relationship between potency and
  the partition coefficient for many types of
  anaesthetic molecules.
• The anaesthetic concentration required to induce
  anaesthesia in 50 of a population of animals
  (the EC50) was independent of the means by which
  the anaesthetic was delivered, i.e., the gas or
  aqueous phase.
• These results on lipid-free proteins show that
  the correlation between lipid solubility and
  potency of general anaesthetics is a necessary
  but not sufficient

9
(No Transcript)
10
LIMITATIONS

• Only applies to gases and volatile liquids oil
  gas partition coefficient cant be determined for
  liquid anaesthetics
• Olive oil is a poorly characterized mixture of
  oils

11
Exceptions

• From the Meyer-Overton correlation, alcohols
  should become increasingly potent as the carbon
  chain length increases because the alcohols grow
  more hydrophobic.
• Instead of becoming increasingly potent without
  limit however, at certain chain lengths the
  addition of just one methylene group causes the
  molecule to lose its ability to anaesthetise.
• Decrease in anaesthetic potency in higher members
  of homologous series is CUTOFF EFFECT.eg .
  n-pentane causes anaesthesia but not n- decane
• STEREOISOMERS
• Enflurane and Isoflurane, are isomers with
  identical chemical properties oil gas partition
  coeffecient, but can differ greatly in
  anaesthetic potencies. suggest that potency
  depends on factors other than lipid solubility
• CONVULSANT GASES
• Some lipid soluble compounds are convulsants and
  not anaesthetics

12
Altered Cell Membrane Permeability

• 1907/1909 HOBER AND LILLIE
• Suggested absorbed anaesthetics decrease cell
  permeability
• Cell was rendered less capable to undergo
  depolarization and thus inhibited
• Turned out to be precedent of a modern theory of
  protein receptor

13
Membrane volume expansion

• 1954 MULLINS
• Proposed that potency correlated better with
  volume rather than number of anaesthetic
  molecules dissolved in oil phase of membrane
• Speculated that they fill voids in membrane
  rather than adding to membranes volume

14
Alteration of lipid layer fluidity

• 1968 Metcalf et al
• Demonstrated that Benzyl Alcohol increased
  mobility of membrane components of erythrocytes
• Suggested that anaesthetics act by increasing
  membrane lipid fluidity which in turn perturbs
  membrane protein function

15
Pressure Reversal of Anaesthesia

• The observed pressure reversal of anaesthesia
  and narcosis is one of the most intriguing
  features of the anaesthetic state. DOCUMENTED
  OVER 50 YRS
• Two theoretical explanation for this effect.
• Anaesthetics and pressure act
  on different molecular targets
• The pressure reversal is a mere
  consequence of a general stimulation brought on
  by pressure overcoming the general depression of
  physiological and mental activity caused by the
  anaesthetics
• Anaesthetic potencies of
  various substances are similar among a wide range
  of organisms, the values of the pressures
  required to reverse anaesthesia vary considerably
  between organisms, and in some cases the pressure
  reversal is not observed at all.

16

• An alternative point of view assumes that
  pressure and anaesthesia act antagonistically at
  the same molecular sites,
• The pressure reversal effect is
  intimately related to a general mechanism of
  anaesthetic action.
• Most obvious effects of the increase of pressure
  at constant temperature is a volume reduction
• some authors have argued that
  anaesthetics act by increasing the local volume
  contribution of some crucial target in the
  nervous system. That is the basis of the critical
  volume hypothesis .

17

• In view of the lipid theories, the pressure
  reversal effect is a consequence of the
  observation that higher pressures reverse many
  anaesthetic-induced perturbations of lipid
  bilayers.
• Protein theories, that assume direct binding of
  anaesthetics to protein sites within ion channels
  or at receptor sites, account for pressure
  reversal through dislocation/dissociation of the
  anaesthetic molecules from their usual targets,
  or modification of the target action

18
Critical volume hypothesis

• 1973 MILLER
• Based on theories of lipid solubility pressure
  reversal of anaesthesia ,he postulated that
  anaesthesia occurs when they expand membrane
  volume beyond a critical amount by 1.1
• Changes in membrane volume compresses ion channel
  and alters function
• Increases in membrane thickness alters neuronal
  excitability by changing potential gradient
  across the membrane

19
Phase Transition Theory

• 1977 TRUDELL
• Speculated that during membrane excitation ionic
  channel protein under go conformational changes
  increases lateral dimensions of protein and
  opens ionic channel
• Normally, membrane lipid near ionic channel
  exists in fluid state Compact
  Gel state during depolarization
• Small increases in membrane fluidity large
  decrease in lateral compressibility of bilayer
  prevent conformational changes in ionic channel
  inhibits membrane excitation
• Anaesthetics enhance membrane disorder by
  increasing fluid to gel ratio interfering with
  ability to open/close channels
  
  

20
Lipid theories

• The MeyerOverton observations - earliest and
  still the best correlation
• Those findings led to a general theory that
  anaesthetics dissolve in the lipid fraction of
  the cell membrane, thus altering the
  physiological properties.
• Modern variants of the lipid theory developed
  related membrane properties that could be
  relevant to anaesthesia, including volume
  expansion and lateral surface pressure, membrane
  fluidity and thickness, and surface tension
  effects.

21

• Such an altered state of the membrane lipids
  might then change the activity and function of
  integral membrane proteins, including
  ion-channels, thereby inducing anaesthesia.
• Modern lipid theories often postulate such an
  indirect mechanism for the occurrence of
  anaesthesia/narcosis.
• The main drawback of these models lies in the
  observation that the anaesthetic concentrations
  needed to produce relevant changes in membrane
  lipid properties would be highly toxic to the
  organism.

22
Macromoleculewater interface theories

• Many of the gas phase species that exhibit
  anaesthetic properties also form crystalline
  hydrates.
• PAULING proposed that hydrated anaesthetic gas
  molecule clathrate can stabilize a membrane /
  occlude essential pores- interferes with
  depolarization
• MILLER postulated that interaction between water
  and anaesthetic molecule results in an iceberg
  which stiffens up membrane prevents neuronal
  transmission
• Anaesthetic gas molecules might then occupy
  structure-determining cavity sites within dynamic
  ice-like liquid water clusters.

23
Limitations

• Neither Clathrate nor iceberg can be formed in
  ambient pressure and body temperature
• Typical gas clathrate hydrates are not stable
  under physiological pressuretemperature
  conditions.
• Additional factors were considered that could
  increase the clathrate stability in a
  physiological environment

24
Protein Theories

• Go back to the late 19th century
• The remarkable finding in1993 Franks and Lieb
  that the soluble protein of firefly luciferase -
  a good model for anaesthetic action represented a
  key advance in the field.
• Detailed analysis of anaestheticluciferase
  interactions led to the suggestion that
  anaesthetic molecules compete with substrate
  luciferin molecules for binding to the protein
  hydrophobic pocket.

25
Contd

• It is now thought that ion channels and
  neurotransmitter receptor sites formed from
  protein complexes embedded within the neuronal
  cell membranes constitute the primary sites of
  anaesthetic action.
• Discovery of the stereospecificity of certain
  anaesthetics and their optical isomers, which are
  equally soluble in lipids, supports a
  protein-based theory of anaesthetic action.
• Opiates can be antagonized by naloxone,
  benzodiazepines by flumazenil and
  non-depolarizing muscle relaxants by neostigmine
  or edrophonium.
• Therefore, in theory it should be possible to
  antagonize i.v. anaesthesia induced by a
  combination of opiate, benzodiazepine and muscle
  relaxant by giving a combination of antagonists
  for each substance.

26
Contd

• No effective antagonist for inhalation
  anaesthesia has been reported so far. This does
  not mean that anaesthesia cannot be brought about
  by receptor action.
• Inhalation anaesthetics may simply act at several
  receptors that differ in their pharmacodynamic
  properties and require several different
  antagonists that have still not been found

27
Neurophysiological Theories

• Suggest that synapses is a likely site of action
• Increase in synapses increases anaesthetic
  sensitivity
• Mechanism of action change in calcium
  permeability at susceptible synapses decreased
  release of neuro transmitter
• Inhalational anaesthetics affect membrane
  structure -decreasing ability of sodium channels
  to open
• LIMITATIONS
• Does not explain how they act
• Changes in EEG SSEP in man differ widely for
  different anaesthetics suggesting multiple sites
  of action
• RAS Is not the sole site
• Only an anatomical description rather than
  molecular expression

28
Biochemical theories

• Volatile agents inhibit mitochondrial respiration
  in a reversible and concentration dependent
  fashion
• Do produce many biochemical effects
• -Alteration in flux of calcium across
  mitochondrial and neural membranes
• -Increase the concentration of GABA in
  synaptic areas by inhibiting degradation-anaesthet
  ic modulation

29
Molecular and cellular theories

• Defined as those theories that are based on
  anaesthetic mechanisms at the molecular or
  cellular level that has been proposed to be
  responsible for generating the state of general
  anaesthesia.
• Enhancement of activity at GABAA receptors is an
  important component, perhaps the only component,
  of the mechanisms that are relevant in
  anaesthesia

30
Genetic studies

• Genetic manipulation of animals increasingly
  used for investigating links between potential
  targets of anaesthesia and their behavioral
  effects
• Screening for mutations in fruit flies
  nematodes identified strain with altered
  sensitivity to anaesthetics but applicability to
  human studies is questionable
• Introduction of specific mutants into native
  genes knock in enables assessment of
  physiological pharmacological role of specific
  proteins
• This method provided evidence for involvement of
  ß2 ß3 sub units of GABA A receptor background
  leak potassium channels in producing immobility
  ,sedation hypnosis
• Highlighted that GABA A ß subunits are
  partially responsible for volatile anaesthetic
  action but crucial for IV agents like Propofol
  Etomidate
• Genetic studies on Drosophila even point to
  voltage-gated sodium channels as factors that may
  affect anaesthetic sensitivity

31
Integrated theories

• Integrated theories of general anaesthesia are
  theories that do not treat the several components
  of general anaesthesia independently but provide
  an integrating basis for their explanation.

32
Targets of Anaesthetic Action

• Molecular targets
• Depending on the physicochemical nature of the
  anaesthetic ,they may prefer
• Within the bilayer, the interface
  between the lipid and the aqueous phase
• Between the lipid and the membrane
  protein
• Hydrophobic core of the lipid bilayer
  itself
• Protein-binding sites exposed to the
  aqueous phase
• Inside membrane proteins or in
  the lumen of ion channels
• Within aqueous domains or water
  channels.

33
(No Transcript)
34
Contd

• May bind within the core of the membrane protein
  itself, between hydrophobic -helices and
  hydrophobic or lipophilic pockets
• May interfere in the interactions between
  subunits of a protein or between different
  proteins
• Interest is focused on anaesthetic interaction
  sites that are amphipathic or lipophilic, i.e.
  sites that have some polar components besides a
  hydrophobic component.

35
Contd

• All members of the main families of ion channels
  and their many subtypes are affected by
  anaesthetics
• Sodium channels, Potassium channels,
  Calcium channels (both voltage and
  ligand-sensitive),
• Glutamate receptors N-methyl-D-aspartate (NMDA),
  -amino-3-hydroxy-5-methylisoxazole propionic acid
  (AMPA) and kainate
• Novel P2X receptors, n AChR
• 5-HT3 receptor channels, GABA A receptor
  channels and glycine receptor channels.

36
Voltage gated ion channels

• Na K channels - Required for action
  potentials
• Insensitive to volatile agents required 8 times
  of halo that of producing anaesthesia- HAYDEN
• Voltage Dependent calcium channel couple
  electrical activity to cellular function
  relatively insensitive -inhibited by volatile
  agents 2-5 times required for anasthesia

37

• Ligand Gated Ion Channels
• Fast excitatory inhibitory neurotransmission
• Glutamate activated ion channels
• 3 types- AMPA, KAINATE - insensitive to
  halothane /sensitive to barbiturates
• NMDA receptors
• - Ketamine is a potent /selective
  inhibitor
• N2O, XENON inhibit excitatory NMDA Glutamate
  transmission
• Glutamate receptors Glu R3 inhibition ,GluR6
  enhanced

38
GABA Activated Ion Channels

• Barbiturates, anaesthetic steroids,
  benzodiazepines, propofol , etomidate ,volatile
  agents modulate GABA A receptor
• Volatile agents alter /potentiate Ligand binding
  in GABA A receptor
• Volume of anaesthetic binding sites on a sub unit
  of GABA A receptor is between 250 370 cubic
• Xenon, Nitrous oxide and Cyclopropane, have
  little effect on GABA -A receptors.
• Genetic studies and evidence from brain imaging
  do not support an exclusive role for GABA - A
  receptors in anaesthesia, although they support
  the hypothesis that the in vivo effects of
  anaesthetics are mediated at least in part
  through GABAergic mechanisms

39
GABA A receptor
40
Contd

• OTHERS
• Volatile agents stabilize the Ach receptors in a
  - conformational stage inactive state
• These receptors play a role in behavioral
  physiological effects
• Have a agent specific effect on 5HT 3 receptors
• Glycine receptor- chloride sensitive ion channel
  potentiated by ? Propofol, pentobarbitol

41
Subcellular targets

• Anaesthetics act on axons and dendrites and
  presynaptic and postsynaptic membranes as well as
  on the somatic membranes of neurones and glia.
• They act on many intracellular structures, such
  as the neurotransmitter release system, the
  calcium homeostasis and buffering system,
  second-messenger cascades and mitochondria.

42

• Cellular targets
• Glial cells,skeletal and cardiac myocytes,
  endocrine cells and cells of the immune system
  are also targets.
• Local microcircuitsAnaesthetic acts on
  microcircuits within slices from dorsal root
  ganglion, spinal cord, thalamus,
  hippocampus,cortex and cerebellum as well as in
  neuronal networks grown in culture.

43
Systems

• Inhalation anaesthetics, to a greater extent than
  i.v. anaesthetics, affect all areas of the CNS.
• Imaging studies indicate that a number of
  discrete brain structures are related to the
  effects of anaesthetics,
• spinal cord, brainstem, cerebellum,
  midbrain and thalamus, midbrain reticular
  formation, basal ganglia, superior frontal gyrus,
  anterior cingulate gyrus, posterior cingulate,
  basal forebrain, insular cortex, prefrontal
  cortex, parietal and temporal association areas,
  occipitoparietal association cortices and
  occipital cortex..

44
Contd

• Functional imaging techniques are beginning to
  help identify key brain structures that appear to
  play important roles in the different clinical
  endpoints produced by anaesthetics.
• The peripheral nervous system also provides
  targets for anaesthetic actions, as does the
  endocrine system and the immune system.

45
Unitary hypothesis

• Overton was a proponent of the unitary hypothesis
  when he stated that, it is highly probable that
  the mechanism of ether or chloroform narcosis,
  for example, remains substantially the same in
  the ganglia cells, the ciliary cells, and in the
  plant cells as well.

46
Contd

• Halsey discussed the unitary hypothesis for
  inhalation anaesthetics (equivalent to Overtons
  non-specific narcotics) three-quarters of a
  century later
• Stated that a unitary hypothesis of
  anaesthesia did not require a single gross site
  of action, as there was evidence for several
  gross sites.
• For him, the unitary hypothesis required identity
  of action at the molecular level.
• Thus, the first possible model was never
  seriously advocated.

47
Contd

• Another example of a unitary hypothesis is the
  GABA hypothesis, -- states that enhancement of
  activity at GABA A receptors is an important
  component, perhaps the only component, of
  relevant mechanisms in anaesthesia.
• Results obtained from genetic studies are now
  being used as evidence that the unitary
  hypothesis can be dismissed in the nematode.

48
Multisite hypotheses

• Allow that many molecular mechanisms may cause
  one or many neuronal lesions, do not hold that
  all anaesthetics should show the same correlation
  between the anaesthetic endpoint and the
  mechanism-related endpoint.
• Thus, if certain inhalation anaesthetics do not
  have much effect on GABA A receptors, this does
  not imply that GABA A receptors are not important
  molecular players in the clinical components of
  anaesthesia.
• Studies of proteins, second-messenger
  signalling, the spinal cord, brain slices,
  genetics and functional imaging all come up with
  data consistent with multisite theories of
  anaesthetic action

49
High pressure effects in anaesthesia and narcosis

• X-ray crystallography has been used to
  investigate the incorporation of species like Xe
  in hydrophobic pockets within model ion channels
  that may account for pressure effects on neuronal
  transmission.
• Magnetic resonance imaging techniques are
  providing tomographic three-dimensional images
  that detail brain structure and function, and
  that can be correlated with behavioural studies
  and psychological test results.
• Voltage-sensitive dye (VSD) imaging studies on
  brain slices provide time-resolved images of the
  dynamic formation and interconnection of
  inter-neuronal complexes.

50
xenon anaesthesia

• Use of Xe results in remarkable cardiovascular
  stability, rapid onset and offset of its action
  resulting from its extremely low bloodgas
  partition coefficient, neuroprotection and
  profound analgesia
• Kr shows anaesthetic effects at higher pressures
  than Xe
• Biochemical studies combined with crystallography
  and molecular dynamics simulations indicate that
  Xe, Kr and N2 could occupy hydrophobic sites or
  pockets within ion channels such as those
  associated with the excitatory glutamate (NMDA)
  neurotransmitter receptor complex

51
Conclusion

• So far, the search for unitary or simple
  mechanisms of anaesthesia has failed -- not a
  consequence of a lack of attempts, but seems
  rather a reflection of its complexity.
• Considering the many different anaesthetic
  effects that have been discovered in vitro and in
  vivo, there are two ways of responding.
• One is that the search for simple mechanisms
  should be continued in order to obtain proof that
  only a few anaesthetic sites and actions are
  really relevant and that the others do not
  matter.

52

• Alternatively, integrated explanations should be
  sought that reconcile many simultaneous
  anaesthetic targets and actions with a still
  functional organism.
• More attempts have to be made to open the black
  boxes.
• Until such networks can be identified and in
  vitro mechanisms tested in these networks in
  vivo, it seems futile to speculate on the
  relevance of in vitro mechanisms for general
  anaesthesia.

53
THANK YOU